1. Field of the Invention
The present invention relates to a crystal diameter measuring device that measures the diameter of the growing portion of a single crystal grown with the pull method (Czochralski technique).
2. Description of the Prior Art
As shown in FIG. 7, the single crystal 36 is grown by immersing the seed crystal 34 in a melt within a crucible and then by pulling it up while the seed crystal 34 is held at the lower end of the pulling shaft 30 by the seed holder 32. In the single crystal 36, a cone-shaped portion 36c is grown following the neck 36t that is grown adjacent to the seed crystal 34.
When pulling an Si single crystal out of an Si melt, volatile SiO is formed due to the reaction between the quartz crucible and the Si melt and this is deposited on the brim of the quartz crucible, the inner wall of the chamber 10, the pulling shaft 30 and the single crystal itself 36. The SiO that is deposited on the pulling shaft 30, which is elevated while rotating, is then scraped off by the ring gasket which provides an air tight seal on the upper lid, and falls down into the melt, causing a defect in the single crystal 36 being formed there.
In order to deal with this problem, a method has been disclosed for example, Japan Patent First Publication No. 64-650865 in which a rectification cylinder 38 is suspended concentrically with the pulling shaft 30, approximately 5 to 100 mm above the surface of the melt and Ar gas flows down from above into the rectification cylinder 38, to expel the SiO, evaporated from the surface of the melt, along with the Ar gas through the lower section of the chamber. The inner diameter of the rectification cylinder 38 is set so that the minimum distance between the inner surface of the rectification cylinder 38 and the surface of the single crystal 36 is 5 to 100 mm.
A window 40 is provided at the lower section of the rectification cylinder 38. The rectification cylinder 38 may be formed of, for example, graphite and the window 40 may be formed of quartz. Through this window 40, the single crystal 36 is recorded with a CCD camera or the like and by processing the image, the diameter D of the luminous ring 44 formed at the solid-liquid interface is measured.
However, if the width of the window 40 is less than the crystal diameter D, the diameter D of the luminous ring 44 cannot be directly measured by scanning the image in the horizontal direction.
In such a case, the crystal diameter D can be measured by using the method disclosed in Japanese Patent First Publication No. 63-112493. In this method, the crystal diameter D is calculated, by scanning the image in the direction of the axis of the crystal 36 and detecting the position P on the luminous ring 44. The crystal diameter D is calculated from position P when the level of the melt surface is constant and crystal diameter D is calculated from the level of the melt surface and the position P when the level changes.
When the image is scanned from top to bottom in the direction indicated with the arrow A, the area 46A around the luminous ring 44 on the cone-shaped portion 36c becomes light due to the mirror reflection from the tilted face of the cone-shaped portion 36c during its formation, resulting in the position PU being erroneously detected as the position P. Consequently, the crystal diameter D cannot be measured accurately. During growing a cylindrical body portion whose diameter is more or less consistent throughout, this problem does not occur.
Also, when the image is scanned from bottom to top in the direction indicated with the arrow B, the position PD is erroneously detected as the position P because of the strong reflection from the area 46B on the melt surface. And just as in the case of downward scanning described above, the measured diameter is not accurate.
The position and size of the light area 46A change as growing the crystal and the position and size of the light area 46B change with the change in the melt surface level.